Illustration of the PSF shapes in the three EPIC instruments at different off-axis and azimuth angles in overlapping observations and of the parameter determination in the simultaneous maximum-likelihood PSF fit. The normalised PSFs – shown here for 1 keV in a linear intensity scale – are scaled to the count rate in the respective image within the detection fit. Position and extent of a source are the same in all observations, while the count rates are fitted in each image. Observation-level and stacked parameters are derived from the parameters of the contributing images (Traulsen et al. 2019).